Thin films of metals and semiconductors (e.g., metal chalcogenides such as CdSe and CdTe) are assuming vital roles in advanced technologies such as nanotechnology. Improved performance of thin-film-based devices, such as nano-scale sensors, magnetic storage media, nano-scale optical devices, and the like, can often be achieved through precise control of the film structure on an atomic to nanometer scale.
Methods of film processing can be divided into two major categories: those carried out in a gas phase (i.e., “dry processes”) and those carried out in a liquid phase (i.e., “wet processes”). The dry processes include, for example, techniques such as vacuum evaporation, chemical vapor deposition (CVD), molecular beam epitaxy (MBE), and sputtering. These techniques generally require high vacuum and/or high temperatures to produce gaseous precursor molecules or atoms, which are then condensed onto a substrate to form a film. Because unwanted chemical species can generally be excluded from the system, the gas-phase techniques can provide high precisely controlled film growth and highly pure materials, however such techniques generally require, expensive, specialized equipment.
Among the wet processes, methods employing chemical and electrochemical reactions in water and in non-aqueous solvents have been used to fabricate structured, thin films. Useful wet methods include chemical bath deposition (CBD), and electrochemical deposition (i.e., electrodeposition or ED). Electrodeposition is widely used for the preparation of decorative and functional coatings, in electronics manufacture, and for the electrochemical fabrication of micro-electromechanical systems. Advantageously, ED also is compatible with relatively simple and inexpensive processing equipment.
With recent developments in nanotechnology, nanostructured thin films have garnered increased interest. Template-based synthesis is one of most efficient approaches to preparing nanostructured thin film materials such as metal and semiconductor nanowire arrays, and the like. Electrodeposition of a metal or semiconductor within a porous template provides a flexible and easily controllable way to fabricate continuous, macroscopic nanostructured arrays as thin films. The electrochemical deposition of a metal occurs at the surface of an electrode and the nanostructured metal grows to fill the pores of the template. The nanostructured thin film can then separated from the template, if desired. So-called “hard” templates, such as alumina and polycarbonate, and “soft” templates, such as liquid crystal materials, have been utilized as templates for electrodeposition of metals to form metal nanostructured materials.
A variety of nanostructures materials have been electrochemically deposited within porous alumina and polycarbonate templates providing arrays of nano-scale wires (i.e., nanowires) having diameters of about 20 to 250 nanometers and with lengths of about 1 to 10 millimeters. Nanostructured materials which have been prepared by theses template-directed methods include nanowires of metals such as Au, Ag, Co, Cu, Ni, Pd, and Pt, semiconductors such as CdS, CdSe, InP, GaAs, and conductive polymers.
Colloidal semiconductor nanocrystals, passivated with organic surfactants and size-selected to a very high degree, spontaneously precipitate out of solution and form three-dimensional arrays of nano-scale crystals (i.e., quantum dots) In this configuration, the array of quantum dots form a quantum dot “crystal” or superlattice array. The strength of the electronic coupling between adjacent dots can be tuned by variation of the organic passivating coating around the dots. Electron transfer within such superlattice arrays results from inter-dot tunneling, which can limit the usefulness of such materials in solar cells, for example.
Semiconductor thin films have been prepared by electrodeposition. Reported electrodeposition of crystalline, group II–VI semiconductor compounds include, for example, galvanostatic electrodeposition of cadmium selenide from a dimethylsulfoxide (DMSO) solution of a cadmium salt and elemental selenium at a temperature of about 185° C.; and potentiostatic deposition of cadmium selenide from an aqueous solution of a cadmium salt and selenous acid at a pH of about 2–2.5. Electrochemical atomic layer epitaxy has also been used to deposit semiconductor thin films. Group III–V semiconductor compounds have been electrodeposited from aqueous solutions or molten salts.
Mesoporous silica has also been utilized as a template material for preparation of nanostructured materials. Various synthetic methods have been developed to produce mesoporous silica with a variety of morphological and topological characteristics including hexagonal mesoporous materials with parallel arrays of relatively uniform diameter cylindrical pores, as well as cubic mesoporous materials having interconnected pore structures. These methods are typically inexpensive and afford templates with readily controllable pore structures.
A number of different nanostructured materials, such as polymers, metals, metallic alloys, semiconductors, and other inorganic compounds, have been synthesized in mesoporous silica templates utilizing non-electrochemical methods, however, there have been no reports of electrodeposition of metals or metal chalcogenides in mesoporous silica templates.
Metal-containing nanostructured films, such as, metal chalcogenide semiconductor nanomesh or nanowire thin films, are useful for preparing nanocrystal-based light-emitting diodes (LEDs) and photoinduced electron transfer devices, for example.
In many cases, nanostructured thin film materials are relatively weak, structurally, which can limit their utility. For example, upon removal of the template from a template synthesized nanostructured thin film, the resulting template-free film can lack mechanical strength and structural integrity (i.e., it can fall apart without external support from a substrate). This can limit the utility of nanostructured thin films in many applications.
There is an ongoing need, therefore, for improved methods of making self-supporting, metal-containing nanostructured thin films, such as metal and metal chalcogenide nanowire and nanomesh thin films, having suitable structural integrity and mechanical strength for use in devices such as solar cells, chemical sensors, and the like. The present invention fulfills this need.